1. Field of the Invention
This invention relates to a method for forming a semiconductor film having amorphous silicon which has a wide optical band gap and is suitable for use as a window material for solar cells. This invention also relates to a method of producing an amorphous silicon solar cell having a high photo-voltaic conversion efficiency at high speeds.
2. Background Art
Amorphous silicon (to be referred to as a-Si:H) films have recently been investigated extensively, and have found wide applications in solar cells, photosensitive drums, scanning circuits for image reading devices and driving circuits for image display devices.
When such a-Si:H films are applied to semiconductor devices such as a window material for solar cells, they desirably have as large an optical band gap (to be sometimes referred to as Eopt) as possible because light having a greater energy than Eopt is shut off by the a-Si:H layer before reaching the optically active layer and is not effectively utilized.
With the prior art, it is difficult to form such a window material from a silicon material alone. For example, a p-type layer formed of monosilane has an Eopt of about 1.6 eV at the highest. Hence, amorphous silicon carbide (to be referred to as a-SiCx) and amorphous silicon nitride (to be referred to as a-SiNy) have been studied, and p-type a-SiCx doped with boron has come into practical use as a window material for hetero junctions.
The a-SiCx and a-SiNy, however, are intrinsically difficult of electrical conduction, and when they are used as a window material, their low electric conductivity gives rise to various problems. If its Eopt is increased excessively by increasing x or y, the loss of an electric current owing to the decrease of electrical conductivity becomes larger than the advantage of taking up light of short wavelengths, and the overall photovoltaic conversion efficiency of the material is decreased.
Thus, the Eopt cannot be increased too much by increasing the x or y value. For example, the balance between Eopt and electrical conductivity in p-type a-SiCx is regarded as best when it is used with an Eopt of 1.9 to 2.0 eV, but at this time, its electric conductivity is as low as about 10.sup.-7 s.cm.sup.-1. Hence, in a photovoltaic converting element such as a solar cell, this window material acts as a considerable resistance component and is very disadvantageous to the improvement of its photovoltaic conversion efficiency.
For the formation of a-SiCx, a carbon source must be used as a raw material. This inevitably results in the deposition of carbon inside a glow discharge chamber, and a new problem of removing the deposited carbon arises. Furthermore, since the conductivity of a-SiCx is greatly affected by its x value, the proportions of the silicon material and the carbon material must be strictly controlled, and the process of making a window material from a-SiCx becomes complex.